The present invention relates to a method for checking the quality of flat workpieces and to a device for carrying out that method, particularly for box making machines used, for example, in the manufacture of folding boxes from a web or sheet matter, like cardboard.
Such machines comprise a plurality of stations arranged one after another enabling them to print the matter, to diecut it according to a required form by means of a plane or rotary tool, to strip the wastes resulting from diecutting, then to check the production quality before collecting the box cuttings, i.e. blanks, into a delivery station.
It is necessary to check the quality when one aims to ensure that production of workpieces will not be spotted by print defects, which are unacceptable for a high quality level. This control is usually automatically processed into the production line. Before stacking of workpieces into the delivery station, this control enables stripping of the blanks that are not in conformity with the minimal printing and/or diecutting quality requirements required for the printing.
Devices operating such controls into a machine for producing packaging are already known. One of said devices, as described in patent CH689770, comprises a camera connected to a device for image processing. The camera operates by registering and storing the position of register marks located on the front waste portion of a sheet. This front waste portion also presents a gripping surface used by a gripper bar for seizing each sheet and conveying the sheets successively from the first station to the last station, by means of an endless gripper bar chain connected to a gripper bar. The camera allows registering an image of the front waste portion where the printing and diecutting register marks are located. By comparing the camera image with a reference image, the device for image processing is then able to determine if allowances related to printing and diecutting quality are required. Although it works satisfactorily, that device is however limited to a register control of colors between themselves and to a positioning control of the print with respect to diecutting. But it does not enable registering print defects located anywhere onto the whole print surface.
Each one of the printing units of the printing station is likely to generate various defects looking like trains, ridges, spots, intensity variations or even non-printed areas, for example. Although limited to the examination of specular surfaces, the device of patent CH692847 registered such defects on the surface of a whole printed web. The defects that are registering are processed on the whole surface of a print format by a camera, and are then compared with the print of a single reference motif, which previously had been recorded. The web print format usually comprises a plurality of blanks evenly distributed with one another in order to reduce the inserted waste as much as possible. The outline of each blank corresponds to a flat box, in a developed shape. The maximal print format is defined by the geometrical dimensions of the printing units cylinders, i.e. by the width and the circumference of the printing cylinders, or by the width of the print substrate, i.e. the sheets or the processed web width.
When a print defect is registered by the camera, an operation has to check the portion of the web spotted with the defect to strip it downstream, usually after it is diecut into separate blanks. It is indeed easier to strip small surfaces which have already been split rather than trying to eliminate a whole sheet or web portion equivalent to the whole format surface. The size of that format is often important and can typically be of about ½ m2 to 1 m2 for example. The diecutting operation is processed either rotatively, by conveying the print substrate between a diecutting cylinder and an anvil cylinder, or flatbed into a platen press equipped with a plane diecutting tool that is vertically and cyclically moved. Such diecutting tools are described with their details respectively in patent application EP1060850 and in patent CH689975.
The stripping of waste portions is usually related to the print quality control within a production line, from which the stripping results thus more usually directly.
At present, one technique is to strip a whole line of diecutting blanks, by means of a swinging flap located across the whole width of the machine. This method is described in patent CH633761. It has a drawback of producing an important useless waste, since the blanks even without any print defect, but located next to the defect blank of the same line, are also stripped.
U.S. Pat. No. 5,235,883 discloses a stripping system including two flaps arranged side by side and occupying the whole width of the diecutting web. It is possible to independently drive one or the other flap so as to strip only the blank having a print defect. This system can satisfactorily work for blanks ranging from medium to large sizes. But, it is not effective when it is used for small sizes blanks, like cigarette packaging, for example. If the web width can indeed comprise up to ten blanks, it would be necessary to provide as many independently movable flaps. One understands that such a device would quickly become too delicate, complicated and even expensive for production. Moreover, since the width and number of the flaps are necessarily related to the width and the number of blanks per line, that device would not be convenient for job changes, e.g., changing from a ten blanks printing to another printing of only six blanks per line.
Another drawback of devices with several stripping flaps arranged side by side is that they provide blanks piles of different heights into the delivery station. If one line of the print format comprises several spotted blanks, those blanks will be automatically stripped, whereas the blanks in the bordering lines will successfully reach the quality control and would thus all be collected. A variable and hazardous blanks number per pile will thus be reached and this fact is unacceptable for manufacturers and customers using said machines.
Another drawback of machines that strip the waste directly inside their production line results from their fast production speed. If those machines can effectively work at high rates, they make the stripping devices appreciably more complicated, as those devices are to be limited to very short reaction times and make the follow-up, by sensors of a blank intended to be stripped. Moreover, the high speed stripping also increases the stuffing risks that should then cause stoppage of the whole producing machine and tend to cause important additional web waste at the time of each re-start. At such speeds, it becomes very difficult to strip, at the right time, the right blank that has been registered upstream by the quality control device as not being conforming.
To obviate this drawback, stripping the two blanks next to the defective one has been considered, to make sure that the defective blank will be certainly stripped. However, this process uselessly increases the waste and increases the drawback related to the variable blanks number per pile at the time of the delivery.
To obviate the inconvenience caused by high speed stripping, one solution was to simply not strip the defective blanks and to manually carry out that operation after delivery of the piles of blanks. Since the quality control system is able to check in which pile of blanks in which a defective blank was registered, this blanks sorting became quickly too long, demanding and quite not very obvious for small and less visible defects. Moreover, the risk of not finding the defective blanks anymore was the more persistent and also did not then ensure a hundred percent defectless quality. Lastly, because of high production rates, ranging from 8,000 to 12,000 boxes per minute, several operators had to be necessarily simultaneously assigned to the manual production sorting of one machine.
If all blanks of a sheet or a printed format are usually the same, it can also happen that they are sometimes slightly different from one another, due to three or four different kind of printings being simultaneously processed with the same printing format. Thus more particularly occurs with cigarettes packaging where a mixture of unlike boxes models are typically found in a carton sold on the market. These boxes can be differentiated due to small distinct indications. Some blanks or boxes mention, for example, “highly injurious to health”, or “smoking causes cancer” or even “smoking is injurious to children of pregnant women”. This mixing of box models in a same cigarette carton is one producers' requirement. That is the reason why the blanks of a same sheet or a same printing format which is intended for processing cigarette packaging will not all be the same but will usually comprise a mixing of some blanks that are slightly different from one another. In such a case, the plurality of models will thus cause new inconvenience while automatically registering print defects, mainly during comparison between not only one main reference model, but between a range of several suitable models slightly different from one another.